The present invention relates to a method for controlling a heating furnace for steel ingots, particularly soaking pit for steel ingots which is charged with a basic fuel quantity for heating the ingots to rolling temperature.
In the soaking pit, the ingots supplied by the steel mill are heated to a rolling temperature as uniform as possible. These ingots had previously undergone a solidification and cooling process where the poured ingots first are cooled in the mold and are stripped in the partially solidified state. Afterwards the stripped ingots are cooled in air and are delivered to the heating furnace into which they are charged. The temperature profile over the cross-section of the ingot at the time of charging in the furnace has a considerable temperature gradient. The center is hottest at 1400.degree. to 1520.degree. C. and sometimes still liquid; the temperature drops to 800.degree.-1000.degree. C. on the ingot surface. This extreme temperature gradient is removed in the soaking pit; the heating time in the furnace is chosen so that at the end of the heating period the ingot has a fairly uniform temperature over the entire cross-section. The temperature level of 1150.degree.-1300.degree. C. depends on the rolling conditions.
The soaking pit is heated by oil or gas. Ordinarily it is constructed so that it can accommodate a number of vertically standing ingots. According to the present state of the art, the furnace is controlled by providing a set point for the furnace temperature. The burners receive a maximum fuel quantity which does not change until the furnace temperature reaches the set point. Upon reaching the set point temperature, the fuel quantity is slowly reduced by the temperature controller and the furnace temperature is held constant. The ingots remain in the furnace at this constant temperature until the operator determines whether ingots in the pit are ready for drawing on the basis of past experience.
This type of control up to draw readiness depends on the heating condition of the furnace and only indirectly on that of the ingots. Therefore, the final roll temperature must be used for correcting the furnace operation, to avoid roll temperatures which are too low or too high.
Accordingly, it is an object of the present invention to provide a method for controlling a furnace of the type described above in order to obtain for the steel ingots at the end of the heating an optimum ratio of the surface temperature to the center temperature. With consideration for the material properties of the ingots, the physical features of the furnace and the disturbances acting on the furnace, it will be possible to select for the transition of the ingot temperatures from the start to the end of the heating time in the furnace either a fixed drawing time for the ingots at optimum fuel consumption, or a floating drawing time for the ingots with an optimum heating period.
Another object of the present invention is to provide an arrangement of the foregoing character which is substantially simple in construction and may be economically fabricated.
A further object of the present invention is to provide a method for controlling a heating furnace for steel ingots, as described, which may be readily maintained in service and which has a substantially long operating life.